PROPOSAL AMENDED:
The global cooldown is now replaced with the following:
Instead of a global cooldown on missions, all missions except for stealing technology have a 15 turn individual cooldown in that city, representing heightened security around that part of the infrastructure.
In addition, a player-specific cooldown is added: only one spy mission per enemy player can be performed in a given city every 15 turns. If you stockpile NP, you can't release it all at once in one fell swoop as that would be too punishing.
And my replacement for the "steal gold" mission is removed. I think this avoids the excesses of having no cooldown, while also avoiding the fatal flaws with the original proposal. To a degree, this also addresses
@Anarcomu's concern about excessive Production sabotage.
I've already said that the cooldown ends after the mission performer's turn, so everyone gets to take turn doing missions.
Moreover, it always takes more than 15 turns to get enough NP for each mission.
No cooldown means the possibility of having local defense and rural infrastructure disabled at the same time, while the city is stuck in a riot with barbarians spawning.
It's also possible for a spy to stockpile a lot of NP and spend them for back-to-back missions (risking the 10% chance of losing everything).
Mission-specific cooldown is a pain to show to the players. They'll need to be on the mission selection screen to see those, which is two menus in and shouldn't be accessed outside of picking a mission.
Losing 50% of your treasury without a cooldown (old system), or 120% of your net GPT (common in current system), is unfun.
Losing 20% every 15 turns is perfectly ok; you can easily make that amount back if you had that much to begin with.
It's just there for thematic reasons, and I think it's an interesting thing to have.
You still have the bonus HP, raw damage reduction, and garrison CS. It's not the same as a freshly settled/conquered city.
Making the city isolated and lose city connection is interesting too, and I'm fine with either.
I think "can't heal" and "+20% bonus against it" is already implied with "Blockaded"?
Cooldown ending after mission performer's turn: this doesn't solve the issue, you could plausibly have to wait 75 turns while four other people ahead of you get to go first. Furthermore, if you don't take your opportunity on that turn, you're screwed. And it's another exploitable Simultaneous Turns mechanic in multiplayer.
Stockpiling NP and using it all at once: that's fair, I amended my proposal by adding a 15 turn
player-specific cooldown.
Mission-specific cooldown being a pain to show: I don't see why this is a problem? The missions take place instantly, and you have to be on the mission select screen to make the selection anyway. You're not locking yourself in for X turns. Furthermore, this is something that could be solved with a UI improvement.
20% every 15 turns is reasonable, I guess.
I think the thematic Tourism effect muddies the waters about the purpose of the mission.
The way CS works, you do disproportionately more damage if there's a big CS differential. It may not be 5 CS, but I still think the city will be too fragile even with those considerations, especially if the garrison has been killed through ranged or aerial bombardment.
All of the effects listed are simply the consequences of a blockade. I edited the text to make that more clear.